1. Field of the Invention
The present invention relates to an image forming apparatus such as copying machine, and more particularly to such image forming apparatus capable of controlling the image forming conditions in response to the original image density to obtain an optimum copy image.
2. Description of the Prior Art
In the image forming apparatus such as copying machine, the adjustment of the copy image density has conventionally been achieved by varying the lighting voltage of an exposure lamp LA1, shown in FIG. 2, by means of a variable resistor VR1 linked to a knob 300 of an operation panel shown in FIG. 1.
However, such conventional method has often required several trial copying operations before an optimum image is obtained, thus leading to an unnecessary waste of copy sheets.
In order to avoid such drawback, there has recently been proposed a system in which the density of the original image is detected in advance and the amount of exposure is regulated according to thus detected image density.
However the sensitivity of a photosensitive drum employed in such image forming apparatus varies according to the number of copying cycles it has been subjected to, so that the control on the amount of exposure alone does not necessarily provide an optimum image density in the copy.